Additive manufacturing techniques and processes generally involve the buildup of one or more materials, e.g., layering, to make a net or near net shape (NNS) object, in contrast to subtractive manufacturing methods. Though “additive manufacturing” is an industry standard term (ASTM F2792), additive manufacturing encompasses various manufacturing and prototyping techniques known under a variety of names, including, e.g., freeform fabrication, 3D printing, rapid prototyping/tooling, etc. Additive manufacturing techniques may be used to fabricate simple or complex components from a wide variety of materials. For example, a freestanding object may be fabricated from a computer-aided design (CAD) model.
A particular type of additive manufacturing is commonly known as 3D printing. One such process commonly referred to as Fused Deposition Modeling (FDM) or Fused Layer Modeling (FLM) comprises melting a thin layer of thermoplastic material, and applying this material in layers to produce a final part. This is commonly accomplished by passing a continuous thin filament of thermoplastic material through a heated nozzle, or by passing thermoplastic material into an extruder, with an attached nozzle, which melts it and applies the thermoplastic material and applies the melted thermoplastic material to the structure being printed, building up the structure. The heated material may be applied to the existing structure in layers, melting and fusing with the existing material to produce a solid finished part.
The filament used in the aforementioned process may be produced by, for example, using an extruder, which may include a steel extruder screw configured to rotate inside of a heated steel barrel. Thermoplastic material in the form of small pellets may be introduced into one end of the rotating screw. Friction from the rotating screw, combined with heat from the barrel may soften the plastic, which may then be forced under pressure through a small round opening in a die that is attached to the front of the extruder barrel. In doing so, a “string” of material may be extruded, after which the extruded string of material may be cooled and coiled up for use in a 3D printer or other additive manufacturing system.
Melting a thin filament of material in order to 3D print an item may be a slow process, which may be suitable for producing relatively small items or a limited number of items. The melted filament approach to 3D printing may be too slow to manufacture large items. However, the fundamental process of 3D printing using molten thermoplastic materials may offer advantages for the manufacture of larger parts or a larger number of items.
In some instances, the process of 3D printing a part may involve a two-step process. For example, the process may utilize a large print bead to achieve an accurate final size and shape. This two-step process, commonly referred to as near-net-shape, may begin by printing a part to a size slightly larger than needed, then machining, milling or routing the part to the final size and shape. The additional time required to trim the part to a final size may be compensated for by the faster printing process.
A common method of additive manufacturing, or 3D printing, may include forming and extruding a bead of flowable material (e.g., molten thermoplastic), applying the bead of material in a strata of layers to form a facsimile of an article, and machining the facsimile to produce an end product. Such a process may be achieved using an extruder mounted on a computer numeric controlled (CNC) machine with controlled motion along at least the x-, y-, and z-axes. In some cases, the flowable material, such as, e.g., molten thermoplastic material, may be infused with a reinforcing material (e.g., strands of fiber or a combination of materials) to enhance the material's strength.
The flowable material, while generally hot and pliable, may be deposited upon a substrate (e.g., a mold), pressed down or otherwise flattened to some extent, and leveled to a consistent thickness, e.g., by means of a tangentially compensated roller. The roller may be mounted in or on a rotatable carriage, which may be operable to maintain the roller in an orientation tangential, e.g., perpendicular, to the deposited material (e.g., a bead or beads). In some embodiments, the roller may be smooth and/or solid. The flattening process may aid in fusing a new layer of the flowable material to the previously deposited layer of the flowable material. The deposition process may be repeated so that each successive layer of flowable material is deposited upon an existing layer to build up and manufacture a desired component structure. In some instances, an oscillating plate may be used to flatten the bead of flowable material to a desired thickness, thus effecting fusion to the previously deposited layer of flowable material. In order to achieve proper bonding between printed layers, the temperature of the layer being printed upon must cool, and solidify sufficiently to support the pressures generated by the application of a new layer. The layer being printed upon must also be warm enough to fuse with the new layer. When executed properly, the new layer of flowable material may be deposited at a temperature sufficient to allow the new layer to melt and fuse with the new layer, thus producing a solid part.
In the practice of the aforementioned additive manufacturing processes (e.g., a 3D printing process), an extruder, of the type used in near net shape 3D printing, may be designed to operate at a steady (e.g., constant) flow rate in order to produce a steady (e.g., consistent), homogeneously melted thermoplastic bead. However, the print head may move at variable speeds throughout the printing process. As such, a print bead that varies in size may be produced. That is, the print bead may be thicker when the machine is moving slowly, and thinner when the machine operates at a relatively higher speed.
One method employed to maintain print bead size (e.g., thickness) may include controlling a servomotor that rotates the extruder screw within the barrel of the extruder. For example, the rotational speed of the extruder screw may be increased when the machine is moving faster, and, additionally or alternatively, the rotational speed of the extruder screw may be decreased if the motion of the machine motion slows. However, the flow rate from the extruder at any point in time is determined not only by the rotational speed of the extruder screw, but also by the recent history of speed of rotation of the extruder screw. For example, the rotational speed of the extruder screw affects the amount of heat energy generated for melting the flowable material, and consequentially the viscosity of the flowable material. In other words, faster rotation of the extruder screw will result in a greater amount of heat energy being imparted to the flowable material, thereby reducing the viscosity of the flowable material. Changing the extruder screw speed may not immediately change the temperature and viscosity of the flowable material. Therefore, if the extruder screw is servo-controlled to operate at a specific rotational speed for a specific velocity of the print head movement, the resulting printed bead may not be consistent. Thus, a melt pump (e.g., a gear pump) may be used with a similar modified servo-controlled approach which may produce a consistently sized print bead when 3D printing. In this process, the extruder screw is controlled to rotate so that a relatively consistent pressurized pool of molten material may be provided to the inlet side of the gear pump. The gear pump may also pull a volume from the pool of molten material with each revolution of the gears. The gear speed of the gear pump may be adjusted in relation to movement of the CNC machine in order relatively accurately control the width of the bead formed from the molten material at various CNC machine speeds.
In a traditional additive manufacturing machine the extruder may be attached to the melt pump using a connecting device, such as, for example, a clamp or a transition plate. When using a clamp, one end of the clamp mechanism may be attached (e.g., bolted) to the output end of the extruder barrel while another end of the clamp mechanism is attached to an input end of the melt pump. The clamping device draws the two ends tightly together to secure the melt pump to the extruder. When using a transition plate, the transition plate may be attached (e.g., bolted) between the extruder and the melt pump. The transition plate may be drilled and/or tapped to exhibit holes that align with holes in both the output end of the extruder barrel and the input end of the melt pump. As such, any connecting device used will have a channel to allow heated printing material to flow from the extruder, through the connecting device, and into the melt pump. Such an arrangement may be inadequate for printing with a variety of printing materials having varying melting points.
Problems are most prevalent when attempting to change from printing materials (e.g., polymers) having a relatively high melting point (e.g., 750° F.) to printing materials (e.g., polymers) having a relatively low melting point (e.g., 500° F.). For example, changing from printing polyethersulfone (PESU) having a melting temperature of about 720° F. to printing acrylonitrile butadiene styrene (ABS) having a melting temperature of about 450° F., may present challenges. Although the following description will describe the process of changing from one polymer to another polymer, it should be noted that the process may apply similarly to any two printing materials having different melting points. The melting point of thermoplastic polymers used for 3D printing may vary from polymer to polymer. The polymers will begin to soften when heated to a temperature close to their melting point, and the polymers will begin to flow when heated to temperatures at or above their melting point. Polymers used as printing materials tend to be highly viscous at temperatures near their melting point, but become less viscous as their temperature is increased. When heated to temperatures significantly above their melting point (e.g., 100-200° F. above their melting point) the viscosity of the polymers is significantly reduced such that the polymers are almost liquid.
Changing an additive manufacturing process from using a first printing material to a second printing material, different than the first printing material, may be referred to as a “changeover” process. Before and/or during a changeover process, it may be necessary to completely purge the first printing material from the print head via the second printing material before printing with the second printing material. For example, extrusion of the second printing material may push or urge any remaining first printing material from the system. That is, when changing from a printing material having a lower melting point to a printing material having a higher melting point, at least part of the print head may initially contain the first printing material having a lower melting point from a previous printing step which may need to be purged, removed, or otherwise replaced with the second printing material having a higher melting point.
The changeover process may begin by heating the print head to a temperature slightly above the melting point of the second printing material with the higher melting point to process (e.g., melt) the second printing material having the higher melting point. In doing so, the first printing material with the lower melting point remaining in the print head will be heated significantly above its melting point, thus reducing the viscosity of the remaining first printing material until it flows freely. Once the temperature of the extruder is sufficiently high to process the second printing material having a higher melting point, the second printing material is introduced into the extruder. Since the second printing material is heated to a temperature nearer to its melting point, the second printing material may have a much higher viscosity than the remaining first printing material. Thus, the second printing material with a higher viscosity may push the remaining, lower viscosity first printing material out of the print head.
A problem may occur, however, when attempting to change from a first printing material having a relatively high melting point to a second printing material different than the first printing material, having a relatively lower melting point. In this case, at least part of the print head, e.g., a channel extending from the extruder to gears in the melt pump, may be at least partially filled with the first printing material having a higher melting point. In order to soften this material sufficiently to allow the second printing material having the lower melting point to purge the first printing material from the system, the temperature of the print head must be raised above the melting point of the remaining first printing material having the relatively higher melting point. If the second printing material having the relatively lower melting point is introduced in an attempt to purge out the remaining first printing material when the temperature of the print head is significantly above the melting point of the second printing material, the second printing material will be significantly less viscous than the remaining first printing material that the changeover process is trying to remove. As a result, rather than completely purging the remaining first printing material, the second printing material having a lower melting point and low viscosity may pass through the remaining first printing material having a higher viscosity. For example, the second printing material may form (e.g. tunnel, cut, etc.) a passage extending through the remaining first printing material, thereby resulting in a collar, or band of unpurged remaining first printing material left over in the channel.
If the print head is cooled to a temperature near the melting point of the second printing material having the relatively lower melting point, some of the remaining first printing material having the relatively higher melting point may cool to a temperature significantly below its melting point and harden in the print head. Such material remaining in the channel after the changeover process may create a number of problems. For example, left over material may narrow a cross-sectional dimension (e.g., diameter) of the channel thereby reducing the flow of material through the system and/or create uncontrolled flow patterns through the print head. Further such, left over material may introduce contamination into the newer printing material. For example, pieces of the hardened left over material may flake off into the newer printing material. Moreover, the hardened leftover material may insulate pressure or temperature sensors located adjacent to channel which may lead to inaccurate measurements on parameters critical to machine operation. Thus, a method and apparatus are desired to allow the change from printing materials having a relatively high melting point to printing materials having a relatively low melting point, while minimizing any material left in the print head as result of the changeover process.